Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/708

690 Mr Darwin has further endeavoured to give a physical explanation of hereditary transmission by his hypothesis of Pangenesis ; while he seeks for the principal, if not the only, cause of variation in the influence of changing conditions. It is on this point that the chief divergence exists among those who accept the doctrine of Evolution in its general outlines. Three views may be taken of the causes of variation:—

a. In virtue of its molecular structure, the organism may tend to vary. This variability may either be indefinite, or may be limited to certain directions by intrinsic condi tions. In the former case, the result of the struggle for existence would be the survival of the fittest among an indefinite number of varieties ; in the latter case, it would be the survival of the fittest among a certain set of varieties, the nature and number of which would be predetermined by the molecular structure of the organism. b. The organism may have no intrinsic tendency to vary, but variation may be brought about by the influence of conditions external to it. And in this case also, the varia bility induced may be either indefinite or defined by in trinsic limitation. c. The two former cases may be combined, and variation may to some extent depend upon intrinsic, and to some extent upon extrinsic, conditions. At present it can hardly be said that such evidence as would justify the positive adoption of any one of these views exists.

If all living beings have come into existence by the gradual modification, through a long series of generations, of a primordial living matter, the phenomena of embryonic development ought to be explicable as particular cases of the general law of hereditary transmission. On this view, a tadpole is first a fish, and then a tailed amphibian, pro vided with both gills and lungs, before it becomes a frog, because the frog was the last term in a series of modifica tions whereby -some ancient fish became an urodele amphi bian; and the urodele amphibian became an anurous amphi bian. In fact, the development of the embryo is a recapitu lation of the ancestral history of the species. If this be so, it follows that the development of any organism should furnish the key to its ancestral history ; and the attempt to decipher the full pedigree of organisms from so much of the family history as is recorded in their development has given rise to a special branch of biological speculation, termed phylogeny.

In practice, however, the reconstruction of the pedigree of a group from the developmental history of its existing members is fraught with difficulties. It is highly probable that the series of developmental stages of the individual organism never presents more than an abbreviated and condensed summary of ancestral conditions; while this summary is often strangely modified by variation and adaptation to conditions ; and it must be confessed that, in most cases, we can do little better than guess what is genuine recapitulation of ancestral forms, and what is the effect of comparatively late adaptation.

The only perfectly safe foundation for the doctrine of Evolution lies in the historical, or rather archaeological, evidence that particular organisms have arisen by the gradual modification of their predecessors, which is fur nished by fossil remains. That evidence is daily increasing in amount and in weight ; and it is to be hoped that the comparison of the actual pedigree of these organisms with the phenomena of their development may furnish some criterion by which the validity of phylogenetic conclusions, deduced from the facts of embryology alone, may be satis factorily tested.

Bibliography.—Haeckel, Generelle Marphologie ; H. Spencer, Principles of Biology.

LIMITS AND CLASSIFICATION OF THE VEGETABLE KINGDOM.

The fundamental difference which separates the vegetable kingdom from the animal kingdom is to be found in the modes of nutrition which obtain in each. If we compare a plant and animal reduced to their simplest terms, and consisting, therefore, in each case of a single cell, i.e., of a minute mass of protoplasm invested with a cell-wall, while the unicellular plant draws its nutri ment by simple imbibition through the cell-wall from the surrounding medium a process which implies that all its nutriment passes into it in a liquid form the unicellular animal is able to take in solid nutriment by means of interruptions in the continuity of the cell-wall, and is also able afterwards to reduce this solid food, if of a suitable composition, to the liquid state. And not merely is there a difference of this kind in the mode, there is also one no less important, although less general, in the materials of nutrition. While under present terrestrial conditions those substances, or chemical combinations, which are required for the nutrition of animal organisms, are, as far as we know, nowhere spontaneously produced that is to say, nowhere apart from the influence of living organisms materials derived wholly from the in organic world are sufficient to sustain directly nearly the whole of vegetable life, and therefore, indirectly, of all other life as well. Roughly speaking, while plants are able to use for the purposes of nutrition binary compounds, such as carbon dioxide (CO.,), water (H 2 0), and ammonia (Nil), animals are essentially dependent on the same elements as enter into these compounds, but mostly in a higher state of chemical aggregation than the binary. Plants, therefore, are the &quot; hewers of wood and drawers of water &quot; for other living things. And this property which they so largely possess of constructing, from materials not directly available for animal nutrition, substances which are so, is found to be uniformly attended with the presence of a peculiar green colouring matter, known as chlorophyll, with which a portion of the protoplasm of their cells is tinged. Many plants, however, such as the whole group of Fungi, as well as some flowering plants, draw their nutriment from compounds derived from other organisms, and therefore in a higher state of chemical aggregation than those the green plants make use of. So far they approach animals in the mode of their nutrition. At first sight it might seem a probable hypothesis that the part played by green plants is one which has always been filled by them from the earliest appearance of life upon the earth. It must, however, be noticed that the presence of chlorophyll in the organism depends upon a specialization of some only of its constituent cells, and of part only of the protoplasmic contents of those cells. The inference, which appears to be justified by general biologi cal principles, is that such a specialization is not a thing of primary origin, but has been gradually attained. We are thus, therefore, led to the supposition that the very earliest plants probably belonging to the same stock as the very oldest animals were destitute of chlorophyll, and were nourished, as Fungi are now, by the imbibition of sub stances fitted for their nutrition, but which, in the condi tions that accompanied the first appearance of life upon the earth s surface, were produced independently of any organisms. The development of chlorophyll would, there fore, on this view, have to be regarded as a later acquirement. It is necessary to bear some considerations of this kind in mind in order to clearly apprehend the relation to one another of the different phases of nutrition which the vegetable kingdom includes. The plants, for example, which 